Pure Enantiomeric Forms Research Articles

Linotrins: Omega-3 oxylipins featuring an E,Z,E conjugated triene motif are present in the plant kingdom and alleviate inflammation in LPS-challenged microglial cells

Alpha-linolenic acid (ALA), an essential omega-3 polyunsaturated fatty acid found in plants, exerts neuroprotection and anti-inflammatory effects in chronic and acute CNS disease models. However, the underlying mechanisms are not yet understood. Since ALA is not incorporated into the brain, the observed health benefits may result from some of its metabolites. The putative formation of dihydroxylated ALA derivatives (called linotrins) was recently shown in vitro in the presence of lipoxygenases. However, the in vitro biosynthesis of linotrins was neither stereoselective nor quantitatively efficient for studying their physiological roles as enantiomeric pure forms. Herein, we report the first stereo-controlled synthesis that features regio- and stereoselective hydrometalations of alkynes for assembling the sensitive E,Z,E-conjugated trienes, as well as LC-MS investigations that provide evidence of linotrins occurrence in plants. Moreover, strong anti-inflammatory effects on microglia highlight the potential physiological importance of linotrins and open new perspectives in search of CNS therapeutics.

Orientation and conformation of two [6]carbohelicenes in stretched polystyrene and a thermoresponsive polyaspartate.

Two functionalized [6]carbohelicenes, one of which was also available in its two enantiomeric pure forms, were oriented in stretched polystyrene in CDCl3 , and in a recently introduced chiral thermoresponsive lyotropic polyaspartate (poly(benzyl)0.5 (phenethyl)0.5 -L-aspartate) in C2 D2 Cl4 . From the resulting 1 H,13 C residual dipolar couplings, the helical pitch of a methylated [6]carbohelicene was determined and found to be in agreement with theoretical predictions and existing crystal structures (d(C2,C2') ≈ 4.3 Å). For a second [6]carbohelicene with para-methoxyphenyl substituents, a clear conformational preference of the substituents was observed. The orientational properties of the two helicene enantiomers in the chiral polyaspartate are very similar, but both drastically change around 306 K. We suggest this behavior is due to an unusual phase transition in the liquid crystal.

Synthesis of Enantiopure PZM21, A Novel Biased Agonist of the Mu-Opioid Receptor.

PZM21 (1) was recently reported as a biased agonist of the mu-opioid receptor (MOR) with improved antinociceptive effects but reduced side effects than traditional opioid-based analgesics. The original synthesis of PZM21 with the desired (S,S) configuration required the separation of diastereomeric mixture in the final step using chiral HPLC. We have designed a concise synthesis of 1 in the enantiomeric pure form starting with commercially available L-alanine and via a chiral aziridine as a key intermediate. The final product 1 as the (S,S) diastereomer was obtained in 7 steps in 22.5% yield from L-alanine. This synthetic strategy could be readily applied to the development of PZM21 analogs at the thiophenyl position.

Chiral Separation in Preparative Scale: A Brief Overview of Membranes as Tools for Enantiomeric Separation

Given the importance of chirality in the biological response, regulators, industries and researchers require chiral compounds in their enantiomeric pure form. Therefore, the approach to separate enantiomers in preparative scale needs to be fast, easy to operate, low cost and allow obtaining the enantiomers at high level of optical purity. A variety of methodologies to separate enantiomers in preparative scale is described, but most of them are expensive or with restricted applicability. However, the use of membranes have been pointed out as a promising methodology for scale-up enantiomeric separation due to the low energy consumption, continuous operability, variety of materials and supports, simplicity, eco-friendly and the possibility to be integrated into other separation processes. Different types of membranes (solid and liquid) have been developed and may provide applicability in multi-milligram and industrial scales. In this brief overview, the different types and chemical nature of membranes are described, showing their advantages and drawbacks. Recent applications of enantiomeric separations of pharmaceuticals, amines and amino acids were reported.

Theoretical Investigation of the Enantioselective Complexations between pfDHFR and Cycloguanil Derivatives

Point mutations in Plasmodium falciparum dihydrofolate reductase (pfDHFR), especially the double mutant variant (A16V + S108T), led to ineffective inhibiting by cycloguanil (Cyc). Cycloguanil derivatives showed good inhibiting properties against wild-type and mutant pfDHFR with an inhibition constant as low as the nanomolar level. However, there have been no reports on the stereochemistry of the compounds, and this is important because the pure enantiomeric form of a chiral drug can exert desirable, as well as non-desirable responses on the body or both. In this work, three-dimensional structures of Cyc derivatives in R and S configuration were constructed and optimized using Hartree-Fock/6-31G (d,p). Their structures were docked into the binding pocket of wild-type and double mutant (A16V + S108T) pfDHFR, complexed with nicotinamide adenine dinucleotide phosphate (NADPH). Results indicate that both wild-type and mutant pfDHFR are enantioselective towards enantiomeric Cyc derivatives (R and S configuration).

Novel Enantiopure Sigma Receptor Modulators: Quick (Semi-)Preparative Chiral Resolution via HPLC and Absolute Configuration Assignment.

The identification of novel pan-sigma receptor (SR) modulators, potentially useful in cancer treatment, represents a new goal of our research. Here, we report on the preparation of novel chiral compounds characterized by a 3-C alkyl chain bridging an aromatic portion to a 4-benzyl-piperidine moiety. All of the studied compounds have been prepared both in racemic and enantiomerically-pure form, with the final aim to address the role of chirality in the SR interaction. To isolate and characterize enantiomeric compounds, high-performance liquid chromatography (HPLC) procedures were set up. A systematic analytical screening, involving several combinations of chiral stationary and mobile phases, allowed us to optimize the analytical resolution and to set up the (semi-)preparative chromatographic conditions. Applying the optimized procedure, the enantiomeric resolution of the studied compounds was successfully achieved, obtaining all of the compounds with an enantiomeric excess higher than 95%. Lastly, the absolute configuration has been empirically assigned to enantiopure compounds, combining the electronic circular dichroism (ECD) technique to the elution order study.

Stereochemistry of a Rhododaurichromanic Acid Derivative

A rhododaurichromanic acid A derivative was synthesized from the naturally occurring daurichromenic acid in enantiomeric pure form. Its absolute configuration was elucidated by applying VCD, ECD and DFT calculations.

Determination of relative enantiomer migration order using a racemic sample

We developed a method that enables us to distinguish between the same or the opposite enantiomer migration order (EMO) of two enantiomers of a chiral compound with two different selectors. The method is applicable to racemic samples and thus a standard of the pure enantiomeric form(s) is not required. First, complexation constants and mobilities of complexes of the two enantiomers with the first and second selector are determined. However, for a racemic sample it is not possible to deduce whether the first migrating enantiomer with one selector is the same one as the first migrating enantiomer with the second selector. A specific mixture of the two selectors is designed to resolve this. In case the two enantiomers exhibit the same, respectively the opposite EMO in the two selectors, the mixture does, respectively does not separate the racemic sample. Thus two peaks are detected in the first case, while a single coalescent peak is recorded in the opposite case.We demonstrate the method on a racemic sample of amphetamine. Its relative EMO is determined with three cyclodextrins, heptakis(2,3,6-tri-O-methyl)-β-cyclodextrin, (2-hydroxypropyl)-β-cyclodextrin and heptakis(2,3-di-O-acetyl-6-O-sulfo)-β-cyclodextrin.

Pre-Clinical Pharmacokinetic and Pharmacodynamic Characterization of Selected Chiral Flavonoids: Pinocembrin and Pinostrobin.

Delineate the stereospecific pharmacokinetics and pharmacodynamics of the chiral flavonoids pinocembrin and pinostrobin. Characterize for the first time the stereoselective pharmacokinetics of two flavonoids, pinocembrin and pinostrobin and their bioactivity in several in vitro assays with relevant roles in heart disease, colon cancer, and diabetes etiology and pathophysiology. Chiral flavonoids were intravenously and orally administered to male Sprague-Dawley rats. Concentrations in serum and urine were characterized via stereospecific HPLC or LC/MS. Pure enantiomeric forms of each flavonoid were tested, where possible, to identify the stereospecific contribution to bioactivity in comparison to their racemates. Short half-lives (0.2-6 h) in serum were observed, while a better estimation of half-life (3-26 h) and other pharmacokinetic parameters were observed using urinary data. The flavonoids are predominantly excreted via non-renal routes (fe values of 0.3-4.6 %), and undergo rapid and extensive phase II metabolism. Chiral differences in the chemical structure of these compounds result in significant pharmacodynamic differences. The importance of understanding the stereospecific pharmacokinetics and pharmacodynamics of two chiral flavonoids were delineated.This article is open to POST-PUBLICATION REVIEW. Registered readers (see "For Readers") may comment by clicking on ABSTRACT on the issue's contents page.

ChemInform Abstract: Total Synthesis of an Anticancer Norsesquiterpene Alkaloid Isolated from the Fungus Flammulina velutipes.

The first total synthesis of a norsesquiterpene alkaloid (R)-8-hydroxy-4,7,7-trimethyl-7,8-dihydrocyclopenta[e]isoindole-1,3(2H,6H)-dione, isolated from the mushroom-forming fungus Flammulina velutipes, in both racemic and enantiomeric pure forms, is reported. The (−)-enantiomer of the natural product has been synthesized from the D-(−)-pantolactone chiral pool. The synthesis features a one-pot, three-step reaction sequence comprising an enyne RCM/Diels–Alder/aromatization to construct the desired indane skeleton present in the natural product. Our synthesis further confirms the assigned structure and absolute configuration of the natural product.

Enantiomeric separation of biaryl atropisomers using cyclofructan based chiral stationary phases

Normal phase chiral HPLC methods are presented for the enantiomeric separation of 30 biaryl atropisomers including 18 new compounds recently produced via a novel synthetic approach. Three new cyclofructan based chiral stationary phases were evaluated. Separations were achieved for all but six analytes and the LARIHC™ CF6-P alone provided 15 baseline separations. Effects of polar modifiers and temperature effects also were studied. Apparent thermodynamic parameters were determined by van’t Hoff plots. Preparative scale methods were developed and employed resulting in the first ever isolation of these novel atropisomers in their pure enantiomeric form. Insights into the mechanism of retention and chiral discrimination are presented.

Enantiopurity analysis of new types of acyclic nucleoside phosphonates by capillary electrophoresis with cyclodextrins as chiral selectors

CE methods have been developed for the chiral analysis of new types of six acyclic nucleoside phosphonates, nucleotide analogs bearing [(3-hydroxypropan-2-yl)-1H-1,2,3-triazol-4-yl]phosphonic acid, 2-[(diisopropoxyphosphonyl)methoxy]propanoic acid, or 2-(phosphonomethoxy)propanoic acid moieties attached to adenine, guanine, 2,6-diaminopurine, uracil, and 5-bromouracil nucleobases, using neutral and cationic cyclodextrins as chiral selectors. With the exception of the 5-bromouracil-derived acyclic nucleoside phosphonate with a 2-(phosphonomethoxy)propanoic acid side chain, the R and S enantiomers of the other five acyclic nucleoside phosphonates were successfully separated with sufficient resolutions, 1.51-2.94, within a reasonable time, 13-28 min, by CE in alkaline BGEs (50 mM sodium tetraborate adjusted with NaOH to pH 9.60, 9.85, and 10.30, respectively) containing 20 mg/mL β-cyclodextrin as the chiral selector. A baseline separation of the R and S enantiomers of the 5-bromouracil-derived acyclic nucleoside phosphonate with 2-(phosphonomethoxy)propanoic acid side chain was achieved within a short time of 7 min by CE in an acidic BGE (20:40 mM Tris/phosphate, pH 2.20) using 60 mg/mL quaternary ammonium β-cyclodextrin chiral selector. The developed methods were applied for the assessment of the enantiomeric purity of the above acyclic nucleoside phosphonates. The preparations of all these compounds were found to be synthesized in pure enantiomeric forms. Using UV absorption detection at 206 nm, their concentration detection limits were in the low micromolar range.

Synthesis and antitumor activity evaluation of maleopimaric acid N-aryl imide atropisomers

Maleopimaric acid N-aryl imides (2) and methyl maleopimaric acid N-aryl imides (3) were designed and synthesized. Their atropisomers (A and B) were separated into their enantiomeric pure forms and the anti-proliferative activity was tested against NCI, A549, Hep G-2, MGC-803 and Hct-116 cell lines, respectively. A significant difference in the level of cytotoxicity was observed between R and S conformers. Atropisomers A with an R configuration exhibited significant toxicity (the IC50 values ranging from 7.51 to 32.1μM). Further experiments proved that antitumor activity of 2A was achieved through the induction of cell apoptosis by G1 cell-cycle arrest.

Synthesis and biological evaluation of enantiomerically pure cyclopropyl analogues of combretastatin A4

To evaluate the influence of stereochemistry on biological activities of cis-cyclopropyl combretastatin A4 (CA4) analogues, we have prepared several cyclopropyl compounds in their pure enantiomeric forms. The key reactions in our synthesis are the cyclopropanation of a (Z)-alkenylboron compound bearing a chiral auxiliary, and the cross-coupling of both enantiomeric cyclopropyl trifluoroborate salts with aryl and olefinic halides. Three pairs of cis-cyclopropyl CA4 analogues were evaluated for their potential antivascular activities. The diarylcyclopropyl compounds with SR-configuration (−)-1b, (−)-2b and the cyclopropylvinyl enantiomer (+)-3a with RR-configuration were the most potent tubulin polymerization inhibitors. A correlation was noted between anti-tubulin activity and rounding up activity of endothelial cells. The cytotoxic activity on B16 melanoma cells was in the submicromolar range for most compounds, but unlike the anti-tubulin activity, there was no difference in cytotoxic activity between racemic and enantiomerically pure forms for the three series of compounds. Molecular docking studies within the colchicine binding site of tubulin were in good agreement with the tubulin polymerization inhibitory data and confirmed the importance of the configuration of the synthesized cis-cyclopropyl CA4 analogues for potential antivascular activities.

Differential recognition of d and l-alanine by calix[4]arene amino derivative

Biomolecules that exist in two enantiomeric forms are generally characterized by their physiological action, e.g. l-alanine is a physiological active form of an essential amino acid. The recognition/separation of one of the enantiomers is an important task as they are used as food supplement or pharmacological products where essentially pure enantiomeric forms are required. Therefore, the quantitation of undesirable enantiomers in drug raw material is the challenging task for pharmacists and chemists. Present study demonstrates the differential recognition of l-alanine amino acid by 5,11,17,26-tetrakis-[(N,N-dimethylamino)methyl]-25,26,27,28-tetrahydroxy-calix[4]arene (3). Another characteristic feature of this study is the use of methyl orange as a UV–visible spectrophotometric probe for the determination of stability constant of host–guest inclusion complexes by adopting competitive inclusion method and 1:1 complexation ratio was confirmed by Benesi-Hildebrand equation. Thermodynamics of the recognition have been evaluated that provided the significant distinction for both isomers, i.e. d and l-alanine and it has been deduced that compound 3 may be employed in chromatographic columns for their separation. Thus, the study provides a broad spectrum of its applications in varying fields of analytical and pharmaceutical science.

Influence of a Change in Helical Twisting Power of Photoresponsive Chiral Dopants on Rotational Manipulation of Micro‐Objects on the Surface of Chiral Nematic Liquid Crystalline Films

Herein we report a group of five planar chiral molecules as photon-mode chiral switches for the reversible control of the self-assembled superstructures of doped chiral nematic liquid crystals. The chiral switches are composed of an asymmetrically substituted aromatic moiety and a photoisomerizing azobenzene unit connected in a cyclic manner through methylene spacers of varying lengths. All the molecules show conformational restriction in the rotation of the asymmetrically substituted aromatic moiety in both the E and Z states of the azobenzene units resulting in planar chirality with separable enantiomers. Our newly synthesized compounds in pure enantiomeric form show high helical twisting power (HTP) in addition to an improved change in HTP between the E and Z states. The molecule with a diphenylnaphthalene unit shows the highest ever known initial helical twisting power among chiral dopants with planar chirality. In addition to the reversible tuning of reflection colors, we employed the enantiomers of these five compounds in combination with four nematic liquid crystalline hosts to study their properties as molecular machines; the change in HTP of the chiral dopant upon photoisomerization induces rotation of the texture of the liquid crystal surfaces. Importantly, this study has revealed a linear dependence of the ratio of the difference between HTPs before and after irradiation against the absolute value of the initial HTP, not the absolute value of the change in helical twisting power between two states, on the angle of rotation of micro-objects on chiral nematic liquid crystalline films. This study has also revealed that a change in irradiation intensity does not affect the maximum angle of rotation, but it does affect the speed of rotational reorganization of the cholesteric helix.

Efficient Dibenzo[c]acridine Helicene-like Synthesis and Resolution: Scaleup, Structural Control, and High Chiroptical Properties

Herein, we describe our recent expeditious synthesis of dibenzo[c]acridine helicene-like compounds on a large scale in pure enantiomeric form. This flexible synthesis allows for variation at several positions on the skeleton. Geometrical parameters related to these compounds have been obtained from monocrystal X-ray structure resolution. Additionally, chiroptical parameters have been recorded, highlighting the versatility of this family showing for example optical rotation at 589 nm varying between 135 and 150 deg g(-1)cm(2).

High-Pressure Access to the Δ9-cis- and Δ9-trans-Tetrahydrocannabinols Family

Diels-Alder reactions of a range of 1-(alkoxy/alkyl-substituted phenyl)buta-1,3-dienes with methyl vinyl ketone and methyl acrylate carried out in ethanol as the reaction medium under 9 kbar pressure were investigated. The use of high pressure as the activating method of the Diels-Alder reactions allows the efficient and endodiastereoselective generation of a series of cis-cyclohexenyl-benzene cycloadducts, which are selectively converted into their trans-epimers. The cis-cyclohexenyl-benzenes and trans-cyclohexenyl-benzenes produced are useful precursors for accessing substituted privileged cis-6a,7,8,10a-tetrahydro-6H-benzo[c]chromene and trans-6a,7,8,10a-tetrahydro-6H-benzo[c]chromene skeletons. The total syntheses of Δ(9)-cis-tetrahydrocannabinol (THC) and Δ(9)-trans-THC, through the use of selected Diels-Alder adducts, are described. Finally, a route for obtaining Δ(9)-trans-THC in both enantiomeric pure forms based on the (S)-(-)-1-amino-2-(methoxymethyl)pyrrolidine (SAMP)-hydrazone method is also reported.

ChemInform Abstract: Water-Promoted Claisen Sigmatropic Rearrangement Using Glyco-Organic Substrates. Chiral Auxiliary-Mediated Induction.

Abstract The Claisen sigmatropic rearrangement with glyco-organic substrates is greatly accelerated in water. Moreover, the ability of glucose to function as a chiral auxiliary is illustrated in the preparation of 4-pentene-1,3-diol , obtained for the first time in pure enantiomeric form.

Effect of goniothalamin on the development of Ehrlich solid tumor in mice

In this work the antiproliferative activity of goniothalamin (1), both in racemic and in its enantiomeric pure forms, in a solid tumor experimental model using laboratory animals is described. The antiedematogenic activity displayed by racemic 1 in the carrageenan edema model in mice together with the reduction of Ehrlich solid tumor model suggest a relationship between anticancer and antiinflammatory activities with the antiinflammatory activity favoring the antiproliferative activity itself.